Typically, a reciprocating internal combustion engine includes a cylinder block having cylinder bores, and a cylinder head. The cylinder head is fixed to the cylinder block with head bolts. When fastening the cylinder head to the cylinder block, the cylinder block can be deformed due to fastening force applied to head bolts.
For example, in a cylinder block having closed deck structure, in which upper end of a cylinder block outer wall portion (cylinder outer wall portion) is connected to the upper end of a cylinder block inner wall portion (cylinder inner wall portion) at an upper deck, fastening the head bolts deforms the upper deck. The cylinder inner wall portion is pressed by the upper deck and inclines inward. As a result, the cylinder bores are deformed.
Conventionally, to prevent a cylinder inner wall portion from inclining inward due to deformation of an upper deck, a structure has been proposed in which sections of the upper deck about through holes for head bolts are thickened compared to the remainder, thereby increasing the rigidity of the upper deck (for example, Japanese Laid-Open Patent Publication No. 6-213064).
However, the structure disclosed in the publication No. 6-213064 is designed for increasing the rigidity of the upper deck, thereby reducing the amount of deformation of the upper deck. Thus, to sufficiently preventing the cylinder inner wall portion from inclining inward, the weight of the cylinder block is unavoidably increased, for example, at the upper deck. That is, such a method for increasing rigidity unavoidably increases the weight of a cylinder block, and thus still has plenty of room for improvement.